Mono-stereo control apparatus for an f. m. stereo signal receiver system



LOUGHLI N ETAL MONO-STEREO CONTROL APPARATUS FOR AN F.M. STEREO June l, 1965 SIGNAL RECEIVER SYSTEM Drlglnal Flled Feb. 26, 1962 2 Shee ts-Sheet 1 ummjnn V l l I l l l I l l I i l l d N umOPOmPmQ N .wi

LOUGHLIN ETAL 3,187,103 MONO-STEREO CONTROL APPARATUS FOR AN F.M. STEREO SIGNAL RECEIVER SYSTEM Original Filed Feb. 26, 1962 2 Sheets-Sheet 2 Jun l, 1965 m .mi

EJQLE United States Patent O M' 2 Claims. (Cl. 179-15) This invention relates to circuits which automatically kill or disable the stereo circuits of a stereo recelver kor stereo adapter during monophonic reception, thereby improving the signal-to-noise ratio during monophonic reception. In accordance with the invention, there may also be provided an indicator which automatically indicates the reception of a stereo signal.

This application is a division of copending application Serial No. 175,736, filed February 26, 1962, for Control Apparatus for a Broadcast Stereo Signal Receiver System.

In its limited sense, the phrase FM stereo is one used in the industry to refer to the FM-AM multiplex broadcast signal complying with the standards ou stereophonic signal broadcasting recently adopted by the Federal Communications Commission. ln this form of an FM stereo signal, left and right channel signals L and R are added together to form an yaudio-frequency sum signal (L-i-R) which frequency-modulates the transmitted carrier signal. As such, the transmitted signal is essentially the same as a monophonic FM broadcast signal` rl`herefore, additional components are necessary to be added to the transmitted signal -for stereo reception. The first of these is an audio-frequency difference signal L-R, which amplitude-modulates a subcarrier of 38 kc. The 38 kc. subcarrier is suppressed and the remaining L-R sideband modulation components, lying in a frequency range above the L-i-R signal, frequency-modulate the transmitted carrier. In addition, a 19 kc. continuous wave signal, serving las a pilot subcarrier, frequency-modulates the transmitted carrier, this pilot subcarrier lying between the L-|R signal and the L-R modulation components.

However, for the purpose of the present specilication and claims, the phrase stereo signal is intended to apply to any stereophonic broadcast signal in which additional signal components are added for the purpose of stereo reception, .and it is not intended to have the more restricted meaning it presently has in the broadcast receiver industry.

When an FM stereo receiver is used to receive monophonic signals, the signal-to-noise ratio may be as much as 20 db. poorer than would be the case if the stereo circuits were removed and the receiver functioned as a conventional monophonic receiver. Hence, it is desirable to disable or kill the stereo circuits of stereo receivers during monophonic reception in order to improve the signal-to-noise ratio. The circuits described herein can be used in either FM stereo receivers or in FM stereo adapters and the phrase FM stereo receiver system is intended to apply to both cases.

In addition to the function of stereo killing, another desirable feature for stereo receivers is an indicator which will automatically show the reception of a stereo signal. Frequently when a person is tuning to a signal, it is not immediately apparent which type of signal is being received, especially when the tuner is close to one speaker location and somewhat remote from the other. As will be seen, the two automatic functions of stereo-killing and indication of stereo signal reception are conveniently combined in the circuits described herein.

3%,l37dd3 Patented June 1, 1965 It is, therefore, an object of the present invention to provide apparatus for automatically controlling stereo circuits in an FM stereo receiver system.

lt is also an object of the present invention to provide an indication of stereo reception when such is the case.

Thus, in accordance with the invention, there is provided for use in a broadcast stereo signal receiver system apparatus for controlling the operation of stereo circuits therein .comprising means for deriving a voltage indicative of the presence or absence of stereo signals in a received broadcast signal. The apparatus also ncludes means responsive to the aforementioned indication for deriving therefrom a control effect capable of disabling operation of the stereo circuits in the absence of a received stereo signal and for otherwise, enabling operation thereof. The apparatus also includes means for applying the control eiect to the stereo circuits to so control them. The apparatus finally includes means responsive to the aforementioned stereo indicating voltage for presenting a visual indication of the presence of the stereo signal and for stabilizing the control effect means against possible variations in the control eect during the operation of the stereo circuits.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

FIG. l is a diagram, partly schematic, of an FM stereo receiver system embodying control apparatus in accordance with one form of the present invention;

FIG. 2 is a modified form of control apparatus useful in the receiver system of FiG. 1, and

PEG. 3 is a further modilication of control apparatus constructed in accordance with the present invention.

FIG. 1 receiver systems There is shown in FIG. 1 an FM receiver system adapted for compatible reception of either a conventional FM broadcast signal or an FM stereo signal, the latter of which may be of the -aforedescribed type approved -by the Federal Communications Commission. As shown, the system comprises an antenna lil, a conventional FM receiver ll, and FM stereo adapter 14, and a stereo amplifier and loudspeaker system l5. In addition, control apparatus '16, to be described in detail hereinafter, is coupled to adapter i4 for controlling the stereo circuits therein in accordance with the present invention. Receiver 1li includes the usual radioefrequency (RF) amplier, frequency converter, and intermediate-frequency (IF) ampliiier circuits l2, the output of which is coupled to an FM detector i3 which may be of any well-known type. The multiplex output of receiver 1l, i.e., an output terminal located prior to the usual de-emphasis circuit (not shown) is connected to the input of adapter 1d.

Adapter 14 is of the type utilizing a local oscillator synchronized by the 19 kc. pilot subcarrier to provide a 3S kc. switching signal for dernodulation of the stereo subcarrier signal. More specifically, adapter ld includes an amplier 17 for amplifying the composite stereo signal and for separating out the 19 kc. :pilot subcarrier signal for application to an oscillator and phase splitter circuit i3. The composite stereo signal is passed through a 19 kc. iilter i9 for preventing translation of the pilot subcarrier and a 67 kc. lter 2li for preventing translation of the 67 kc. SCA (e.g., storecast) subcarrier and its adjacent modulation components to a beam-switching demodulator and amplifier and matrix unit 2l. Unit 2l is presently well known in the art. Specilically, it may be of the type shown at page 20 of the June 1961 issue of Audio Magazine. The dernodulated left and right f 3 channel signals at the demodulator-matrix unit 21 are then applied through a de-em-phasis network 222V for applica- `tion to system including ampliliers'23, 24 and lett and right loudspeakers'ZS and 26, respectively.

The oscillator 18 may, in fact, comprise a 19 kc. oscillator having a resonant tank circuit directly synchronized bythe 19 kc. pilot subcarrier and with Va 38 kc. tuned circuit in the output thereoffor selecting and applying the second harmonic of the 19 kc. oscillator signal to the phase splitter therein. As will become clear hereinafter, it is the function of the control apparatus 16 to control the operation of the stereo cir-r cuits 'in adapter 14 by damping out or not damping out the tank circuit in the oscillator portion of unit 13V in accordance with the absence or presence of the stereo signal respectively.

Control apparatus 16 of FIG..1

Thus, in accordance with one vform of the present invention, there is provided control apparatus 16 including means for deriving an electrical signal indicative of the presence or absence of stereoV signals in a received FM broadcast signal. Such means may include amplifier tube 31, to ,which the composite signal from detector 13 is applied through input terminal 28 and coupling capacitor 29. The usual grid leak resistor 3u, cathode biasing network 32, 33, and screen grid bias network 34, 35 are also included. A coupling transformer 36 is inserted in the anode circuit of tube 31 and is tuned to resonate at a frequency of 19 kc. A diode 37, resistor 38, sensitivity control rheostat 39, and capacitor 41 are coupled to the output of transformer 36, and constitute a detector circuit to provide either of two voltage levels, depending on the presence or absence of the 19 kc. pilot subcarrier at the outputof FM detector 13. The ,output of this detector circuit is applied to the control grid of a'DQ-C. amplifier, conventionally including tube 40, grid biasing resistor 42, and an anode load 43. The value of resistor 42 is such that it maintains the grid bias voltage of tube 40 near ground potential in the absence of a stereo signal.

Control apparatus 16 also includes means such as electronic switch 44, responsive to the aforementioned electrical signal which, in this example, isV the amplied voltage levels at the anode of tube 40, for deriving therefrom a control elect capable of disabling operation of the stereo circuits in the receiver system in the absence of a received stereo signal and for otherwise enabling operation thereof. Electronic switch 44 includes diodes 47 and 48, coupled in series from the anode of tube 40 through resistor 49 to the movable tap of a voltage-divider potentiometer 50. A capacitor 45 is coupled across diodes 47, 48 and capacitor 46 is coupled from the anode of diode 48 to ground. Y

Means for applying the control erect developed by electronic switch 44 to the stereo circuits includes capacitor 51, coupled from the common connection of diodes 47, 4S through output terminal 53, to one end of the aforementioned resonant tank circuit in oscillator 18. In addition, there is provided means such as neon indicator lamp 52, connected to the anode of tube 40 and responsive to the voltage'levels on the anode for presentinga visual indication of the presence of the stereo signal and for stabilizing the control eiect during one mode of its operation.

The operation of control apparatus 16 is as follows.

During monophonic reception the entire output of detec- Y tor 13 is applied to input terminal 2S; however, since there is no 19 kc. pilot subcarrier signal present, no signal will appear at the plate of tube 31. This is because the plate load, i.e., transformer 36, is tuned to 19 kc. The grid of tube 40 will be close to zero potential, causing substantial current conduction. The plate of tube 40 will, therefore, be at some low voltagebelow the extinction potential of neon indicator Ylamp 52, which 4 consequently will be out.Y Potentiometer 50 is adjusted so that the voltage at the movable tapis higher than the aforementioned plate voltage toV tube 40, but lower than the maintaining potential of neon lamp S2. It is preferably adjusted to a point midway -between the two values. Thus, since they are forward biased, current will iiow through diodes 47 and 43,' and the tank circuit of oscillator 13 will be heavily loaded by the loading circuit composed Vof capacitors 51, 45, and 46.

During stereo reception, a 19 kc. pilot subcarrier appears (along with other signal components)V at input terminal 28 and is selectedV and amplified by the ampliiiercirouit of tube 31. i It is then rectiiied by diode 37, producing a negative bias at the grid of tube 40 sufficient to raise the plate potential or" tube 40 to the striking voltage of the indicator lamp 52. When the lamp lights, the plate voltage will then drop to the maintaining Voltage of the lamp which, as previously indicated, is higher than the `voltage at the movable tap of potentiometer S0. Diodes 47 and 43, in series, will then be reverse biased, rendering them nonconductive and thereby removing the loading across the tank circuit of oscillator 1S. This enables oscillator 1S to operate in its normal manner for stereo reception. At the Ysame time, the lighting of lamp 52 gives an automatic indication of the presence of a stereo signal which greatly simplies tunparticular set of `circuit constants, the following have provedruseful in the circuit of FIG. 1

Resistor'30 470 kiloms. Resistor 32 V 150 ohms. y Resistor 34.V 47 kilohms. Resistor 38 470 kilohms. Resistor 42 2.2 megohms. Resistor 43 220 kilohms. Resistor 49 220 kilohms. Rheostat 39 100 kilohms (max). Potentiometer 50 100 kilohms (marc). Capacitor 29 5000 picofa-rads. Capacitor 33 4 microfarads. Capacitor 35 16 microfarads. Capacitor 41 1.0 microfarad. Capacitor 45 0.01 microfarad. Capacitor 46 0.01 mcrofarad. Capacitor 51 0.01 microfarad. All Idiodes Type 1N34. Tube 31 Type 6AU8 (pentode fr Y section); Tube 4@ Type 6AU8 (triode section).Y Transformer 36 EL-RAD: X61-279 (with Y 6200 pf. across primary and secondary). Lamp 52 Type NEZ. Voltage Supply B-l- 200 volts.

In FIG. 2 there is `shown an alternative form of control circuit in oscillator 1S of FIG. 1. During stereo reception -a bias will be produced at the grid of tube 4t?, reducing the plate current and relay 54 will release, oscillator 13 Will operate, and indicator lamp 52 will light, visually indicating to the listener the presence of a stereo signal. In an lactually constructed embodiment of the FIG. 2 apparatus, a value of 12 kilohms was used for resistor 43 and a type XZ-366 relay, available from Comar Electric Co., was used for relay 54.

FIG. 3 receiver System There is shown in FIG. 3 an FM stereo receiver system utilizing a diierent type of stereo adapter 314 and a modified control appanatus 316. Adapter 314 is of the type which employs a nonoscillating frequency doubler 5S to produce the 38 kc. subcarrie-r in place of the oscillator 18 of FIG. l. As will be seen hereinafter in accordance with one form of the invention, `control apparatus 3M disconnects the frequency doubler 53 and the (L-R) sideband-supply circuit S6 from the subcar-rier detector and matrix 61 in the absence of a stereo signal and makes the connection in the presence of a stereo signal.

Considering the receiver system in general terms, antenna 1t) and receiver 11 may be identical t-o the corresponding yunits of FIG. 1, -as is also true with the stereo amplifier and loudspeaker system 15. In adapter 314 the lowfrequency information signal (L-i-R) is separated out by iilter 55 and applied to the matrix in unit 61 including resistors 68 and 6g. rlhe high-frequency (L-R) sidebands are lseparat-ed out by lter 55 and applied to terminal 60, at which point the locally derived 38 kc. subcarrier from frequency doubler SS is added via resistor S9 to produce a subcarrier .signal amplitude-modulated by the signal (L-R). Frequency doubler S3 develops and amplities the second harmonic of the 19 kc. pilot subcarrier which is separated `from the received stereo signal by means of a narrow bandpass lter 57. Normally, the reconstituted (L-R) subcarrier signal would be connected to the detector input of unit 61, as shown by the dotted line connection. However, in accordance with the invention, it is supplied to input terminal '79 of control apparatus 316 for control purposes which will now be described.

Control apparatus 316 Although a nonoscillating frequency doubler circuit will inherently have less noise output during monophonic -reception than an oscillator circuit, significant further noise reduction may be obtained in receiver systems of this type by adding the control yapparatus 316, Which automatically disconnects the (LR) circuit and the 38 kc. doubler circuit from the two detectors 62 and 63 during monophonic reception. A further beneiit of control apparatus 316 is that by disconnecting the unit 6l detectors, clipping of the monophonic (L-l-R) signal is prevented, the monophonic output being somewhat higher in amplitude than the stereo output.

Considerinc now the description of cont-rol apparatus 316, it includes means responsive to, for example the 19 kc. pilot subcarrier, for deriving therefrom an indication oi the presence 'or absence of stereo signal-s in the received broadcast signal. This means may include the connection from the plate oi the tube in frequency doubler 5S through input terminal 2S and coupling capacitor 72 to the anode of one section 73a, connected as a diode, of twin triode 73. Resistor 74 is the diode load and the detected signal is coupled 4to the control grid of section 73h, connected as an amplifier, through resistor 75. Capacitor 76 is the rectier circuit filter capacitor and resistor 7S maintains the section '73h grid potential close to zero in the absence of a 38 kc. signal from frequency `doubler 5S. Control apparatus 3116 .also includes means coupled in between terminal 60 and the detectors 62 and 63 and being responsive to the plate voltage of tube section 73h to couple the signal at terminal 60 to diodes 62 and 63 in the presence of a stereo signal and to uncouple said connection in the absence of .a stereo signal. This means comprises electronic switch Si) including diodes 31 and 6 82, D.-C. coupled 4fnom the plate of `tube section 73h to ground through .an adjustable voltage divider in a manner similar to switch 4410i FlG. l, except that the polarity of the diode connections is reversed. In addition, a neon i7ndbicator lamp 89 is coupled to the plate of tube section In operation and during monophonic reception, there will be no signal output from the 38 kc. frequency doubler 58. The grid of section 73b of tube 73 will be close to zero potential, the plate will be at a relatively low D.-C. voltage, and neon indicator lamp 89 Will be out. Potentiometer 85 is adjusted to a potential midway between the aforementioned plate potential of tube section '731: and the maintaining potential of the neon lamp. Diodes 81 and 82, in series, will, therefore, be reverse-biased and the undesired noise signals at terminal 6d will be effectively disconnected from the two diode detectors 62 and 63, thereby preventing any noise from filter S6 or frequency doubler 58 from appearing at the input of audio amplifiers 23 and 24. Furthermore, the diode detectors cannot clip the monophonic signal.

During stereo reception, a signal voltage will appear at the plate of the tube in the 38 kc. doubler circuit 58. This signal Voltage will be rectied by lsection '73a of tube 73, connected as a diode, and will be applied to the grid of tube section 73h. The voltage at the plate of tube section 73b will rise to the striking potential of neon lamp 89 and then drop to the lamps maintaining voltage. Forward current will then tiow through diodes 81 and 82, dropping their impedance to a low level, and the 38 kc. subcarrier signal and the (L-R) sidebands signal will be coupled to detector diodes 62 and 63. Resistors S3 and S4 are inserted to reduce the loading eect of the electronic switch.

It will be appreciated that the electromechanical switch of FIG. 2 may readily be substituted for the electronic switch 8u of FIG. 3, the details thereof being obvious and, therefore, no specific description being necessary. It will also be obvious that the switch may be located at a point subsequent to the subcarrier detectors, for example by employing a pair of switches, one in each channel. However, for a system such as is shown in FIG. 3, Where the same amplifier and loudspeaker system is used for monophonic and stereophonic reception, it Would be necessary to ensure that lsuch a switch arrangement not be so located as to disable the monophonic circuits.

While applicants do not wish to be limited to any particular set of circuit constants, the following have proved useful in the circuit of FIG. 3:

Resistor 74 470 kilohms. Resistor '75 470 lrilohms. Resistor 77 220 kilohms. Resistor 73 3.9 rnegohms. Resistor 33 47 kilohms. Resistor 84 47 kilohms. Potentiometer 85 100 kilohms (max). Capacitor 72 330 picofarads. Capacitor '76 0.1 microfarad. Capacitor 86 0.1 microfarad. Capacitor 87 0.1 microfarad. Capacitor 88 0.1 microfarad. Diodes 81, 82 1N67A.

Tube 73 12AT7. Voltage supply B-jvolts.

While there have been described what are, at present, considered to be the preferred embodiments of this invention, it will be ovbious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. For use in a broadcast stereo signal receiver sys- A 7 tem, apparatus for controlling the operation of stereo circuits'therein comprising:

means for deriving a voltage indicative of the presence or absence of stereo signals in a received broadcast signal; means responsive to said indication for deriving thereyfrom a control effect capable of disabling operation of the stereo circuits in the absence of a received stereo signal and for otherwise enabling operation thereof; means for applying said control eect to the stereo circuits to so control them; and means responsive to said stereo indicating voltage for presenting a visual indication of the presence of said stereo signal and for stabilizing said control effect means against possible variations in said control eiect during the operation of said stereo circuits. 2. For use in a broadcast FM stereo signal receiver system, apparatus for controlling the operation of a carrief-injecting oscillator included in the stereo circuits thereof comprising: f k

first means including an amplier circuit responsive to a signal component appearing only in a received FM stereo signal for providing a lirsrt voltage level f representing the presence of said signal component and a second voltage level representing the absence of said signal component;

second meansresponsive to said rst and second voltage levels for deriving therefrom a variable loading elect capable of damping outoscillations in the resonant circuit of said oscillator in the absence of a received stereo signal and otherwise enabling said oscillations, said second means including a loading circuit and a diode circuit coupled eiectively in yseries with the loading circuit and also coupled to said ampliiier circuit so that said rst voltage level reversebiases the diode circuit and the second Voltage level forward-biases the diode circuit; Y means for coupling said loading circuit to said resonant circuit, whereby said oscillations are damped out only when said diode circuit is forward-biased; and combined voltage regulating and visual indicating Vmeans coupled intermediate said amplilier circuit and said diode` circuit for simultaneous-ly providing an indication or" the presence of -said stereo signal and preventing substantial variations in said reverse bias, whereby phase variations in said oscillations are minimized. Y

References Cited by the Examiner Mergner: VProduct Detector forFM-Stereo, Audio, August 1961, pages 23-25 and 102.

DAVID GQREDINBAUGH, Primm Examiner. Y 

1. FOR USE IN A BROADCAST STEREO SIGNAL RECEIVER SYSTEM, APPARATUS FOR CONTROLLING THE OPERATION OF STEREO CIRCUITS THEREIN COMPRISING: MEANS FOR DERIVING A VOLTAGE INDICATIVE OF THE PRESENCE OR ABSENCE OF STEREO SIGNALS IN A RECEIVED BROADCAST SIGNAL; MEANS RESPONSIVE TO SAID INDICATION FOR DERIVING THEREFROM A CONTROL EFFECT CAPABLE OF DISABLING OPERATION OF THE STEREO CIRCUITS IN THE ABSENCE OF A RECEIVED STEREO SIGNAL AND FOR OTHERWISE ENABLING OPERATION THEREOF; MEANS FOR APPLYING SAID CONTROL EFFECT TO THE STEREO CIRCUITS TO SO CONTROL THEM; 